Method and device for document security by generating multiple reflective and transmissive latent images

ABSTRACT

The present invention relates to a production method and to a device for document security applications including various latent images on each side. The invention comprises: depositing, according to an established pattern, at least one layer of metallized material, forming a holographic element on at least one part of one of the surfaces of a confinement substrate; defining different regions on the surface of the substrate; inducing different alignment directions for orienting a liquid crystal according to the previously defined regions; doping the liquid crystal with at least one dichroic dye; placing the liquid crystal over at least one confinement substrate, covering the holographic element; adding a second confinement substrate, forming a sandwich-type structure; and polymerizing the liquid crystal, forming a sheet.

TECHNICAL FIELD OF THE INVENTION

The present invention is applicable to the field of document securityand more specifically relates to the security features provided byoptical devices, which make the verification of original documentseasier, for example, by means of incorporating a sheet with multiplereflective and transmissive latent images.

BACKGROUND OF THE INVENTION

Methods and devices for security purposes existing in banknotes, IDcards or other similar documents often use directly visible holograms asa verification element.

On the other hand, the growing trend of including transparent windows indocuments of a certain type, such as new national ID cards or Eurobanknotes, has led to the development of new and more visually appealingsecurity measures based on transmissive holographic elements.

For example, patent document ES2337010 discloses transmissive sheets,which when illuminated with polarized light, show one or more images oneach side, while maintaining their transparency at the same time. Incontrast, no image is observed when they are illuminated with naturallight. The images can be B/W, monochrome or multicolor images and theycan be provided with grayscale and/or high resolution. Nevertheless,their application for achieving document security is not immediate formany of the existing solutions, since they entail certain requirementsin terms of flexibility, definition and simplicity of production/usethat not all of them comply with.

Patent document WO 98/52077 discloses devices based on photo-orientedpolymer networks (PPNs) arranged on a substrate and selectively orientedin directions which vary in different regions of the surface. In turn,the PPN layer is covered with another layer formed by cross-linkedliquid crystal monomers. This second layer is optically anisotropic andbirefringent, acting as an optical retarder. The liquid crystal of theretarder layer spontaneously follows the selective orientation dictatedby the PPN. This allows to obtain phase delayed images that can be seenwith the help of a polarizer. As can be seen in the drawings, thestructure thereof is indeed a complex one where three layers are needed,two of them being linear retarder layers created from liquid crystalpolymers (LCPs), and the third layer being a linear polarizer.

The state of the art offers several solutions combining directly visiblefeatures and covert features that can be viewed, for example in patentdocument WO 2008/131852-A1; this document, however, requires the use ofa laser, so it does not relate to direct viewing devices. Othersolutions consider exhibiting different images on each side when theyare illuminated with polarized light. However the embodiment andoperating principle of these devices are complex and they require atleast two retarder sheets with linear retarding patterns and a centralpolarizing film, such as in the above-mentioned patent document WO98/52077 or EP 2543521A1.

Therefore new contributions to the state of the art improving currentsolutions for document security by generating multiple reflective andtransmissive latent images in a new simple, flexible and effectivedirect viewing feature that does not require the use of a laser, wouldbe desirable.

DESCRIPTION OF THE INVENTION

The present invention solves the aforementioned problems by presenting anew feature for document security based on the observation ofholographic images under reflection on one or both sides of thedocument, and the observation of one or more images under transmissionon each side depending on the viewing mode. By making use of theproperties of iridescent and non-iridescent variable optical devices,such as anisotropy, optical birefringence, or the phase transition ofthe liquid crystal material itself, optical effects that can be readilyverified with the naked eye or by means of using simple optical elementsare achieved. Each device can offer at least one latent image on eachside that is only visible when it is observed in transmissive mode underpolarized light. Additionally it shall include a reflective holographicimage on at least one side of the film, although it can also be includedon both sides.

To that end, the present invention proposes a method for producingfeatures for document security purpose including several latent images.The method comprises the steps of:

-   -   a) depositing, according to at least one established pattern, at        least one layer of at least partially metallized material,        forming a holographic element on at least one part of one of the        sides of at least one confinement substrate;    -   b) defining different regions on the surface of at least one        substrate;    -   c) inducing different alignment directions for orienting a        liquid crystal material according to the previously defined        regions;    -   d) doping the liquid crystal with at least one dichroic dye;    -   e) placing the doped liquid crystal on at least one confinement        substrate, covering the holographic element;    -   f) adding a second confinement substrate, forming a        sandwich-type structure;    -   g) polymerizing the liquid crystal, forming a sheet.

According to one of the embodiments of the invention, the differentregions in the substrate are defined depending on the holographicelement deposited on at least one part of the substrate's surface.Therefore, the holographic elements can advantageously be used foraligning the liquid crystal and the dichroic dye, allowing the creationof latent images on the LCP layer between the holographic surfaces,which will determine the alignment pattern of the liquid crystal. Morespecifically, in one of the embodiments of the invention, themicrometric or submicrometric grooves of the holographic motifs canadvantageously be used as alignment surfaces for dye-doped liquidcrystals.

One of the embodiments of the present invention contemplates removing atleast one of the confinement substrates once the liquid crystal has beenpolymerized.

One of the embodiments of the present invention contemplates removingboth confinement substrates once the liquid crystal has beenpolymerized.

The techniques used for defining different regions on the inner surfaceof the confinement substrate may comprise at least one of the followingtechniques:

-   -   photolithography;    -   masks;    -   physical barriers;    -   selective deposition;    -   thermal evaporation;    -   inkjet;    -   micrometric or submicrometric pattern generation;    -   partial (semireflective) metallization which reflects part of        the incident light and transmits the rest;    -   selective reflective or semireflective metallization which        covers a specific area of the device;    -   or a combination thereof.

According to one of the embodiments of the present invention, thedifferent alignment directions can be generated in any directionparallel to the plane of the confinement substrates, wherein thealignment directions induced on one of the faces of a substrate areindependent of those induced in a second substrate and comprise the useof at least one of the following techniques:

-   -   mechanical rubbing of the alignment layer;    -   photoalignment of a photosensitive material;    -   oblique deposition of an aligner material;    -   alignment by means of micrometric or submicrometric pattern;    -   generation of a pattern of interdigitated electrodes on the        surface of the confinement plates oriented in different        directions.    -   or a combination thereof.

It is contemplated that the alignments induced in at least two regionsare not linear with respect to one another. One of the particularembodiments of the invention further contemplates that at least two ofthe induced alignments are orthogonal with respect to one another.

Determining the relative angle existing between the induced alignmentdirections to generate different gray levels in the latent image iscontemplated in one of the embodiments of the present invention.

Adding an RGB color matrix to generate latent images is contemplated inone of the embodiments of the present invention. Additionally, the colormatrix can be arranged such that the regions defined in the sheet aremade to coincide with the pixelation of the matrix on either the outerface of the already polymerized liquid crystal sheet or on the innerface of a protective polymer layer located on the sheet.

According to one of the embodiments of the present invention, differentdichroic dyes can optionally be added in specific regions of the devicesto generate regions of different colors.

A second aspect of the present invention relates to a device fordocument security including various latent images, where said devicecomprises:

-   -   at least one holographic element comprising one or more layers        of at least partially metallized material, deposited on at least        one of the faces of a confinement substrate, forming reflective        holographic images when illuminated (the reflective holographic        image covers at least partially the surface of one side. In the        event that it covers the entire surface of said side, it is only        partially metallized, such that a fraction of the incident light        is reflected, allowing the remaining fraction of the light to be        transmitted in transmissive mode);    -   a liquid crystal sheet, covering at least one holographic        element, doped with at least one dichroic dye oriented according        to the alignment directions induced in different predefined        regions of the inner surfaces of two confinement substrates. The        orientation of the different regions is established according to        at least one alignment pattern and they differ from one another        by an angle smaller than or equal to 90° on at least one of the        faces forming latent images which are visible when illuminated        with polarized light.

Alternatively, the metallized holographic elements can be located inspecific areas of the surface. In this case it can be partially orcompletely metallized, with other regions remaining unmetallized.

It is contemplated that the light used for showing the images intransmissive mode is linearly polarized, although images may becomevisible with a lower contrast using partially polarized light or lightwith circular or elliptical polarization.

The alignment pattern can be established according to the holographicelement deposited on at least one part of the surface of the substrate.

Additionally, the present invention may comprise, according to one ofits embodiments, a protective sheet on both sides of the assembly formedby the liquid crystal sheet and the holographic elements to preserve thedevice.

One of the embodiments of the present invention contemplates differentdichroic dyes distributed throughout the different regions defined inthe liquid crystal sheet, generating different colors.

An RGB color matrix located between the liquid crystal sheet and theprotective sheet is optionally contemplated in one of the embodiments ofthe present invention.

The alignment directions can exhibit relative orientations of 0°, 45°,90° and 135° according to different embodiments of the invention togenerate two non-overlapping monochrome images that are visible intransmission on each side of the sheet.

The alignment directions can exhibit any relative orientation comprisedbetween 0° and 90° according to different embodiments of the inventionto generate grayscale images visible in transmission on each side of thesheet.

According to one of the embodiments, the device of the present inventionmay comprise one or more removable and reusable confinement plates thathave been previously treated to generate an alignment pattern withdifferent regions and orientations in the liquid crystal sheet.

Therefore, the characteristics of the present invention in relation tosecurity features for document security applications made from dichroicdye-doped liquid crystal polymers (LCPs) or reactive mesogens (RMs),involve various advantageous technical effects over the state of theart, such as, for example, the use of substrates with holographicelements that can be selectively metallized in sectors, partiallymetallized or not metallized, such that the patterns can be used toalign the dichroic dye-doped LCP. Once the LCP has been polymerized,this method allows creating reflective and transmissive securityelements integrated in a single device. When the holographic elementsare partially metallized, the patterns shown by reflection on one sidecoincide with the patterns visible in transmission on the opposite side.

The present invention also advantageously uses non-metallizedholographic elements to align dichroic dye-doped LCP layers. When thetransmissive layer is polymerized and removed from the substrates, thepartial or selective metallization allows creating reflective andtransmissive elements in the same device. These security features canshow reflective patterns on one side, if the holographic elements areindeed partially metallized, and the same pattern in transmission on theopposite side.

Another alternative is the use of partially or selectively metallizedholographic elements for aligning dichroic dye-doped LCP layers. Oncethe LCP has been polymerized, it is removed from the substrates alongwith the metallization layer (which was previously located in theholographic element) adhered thereto. The method allows creatingreflective and transmissive security elements integrated in a singledevice. These security elements show reflective patterns on one side, ifthe holographic elements are indeed partially metallized, and the samepattern in transmission on the opposite side.

A final aspect of the present invention relates to the use of partiallyor selectively metallized holographic elements that adhere to dichroicdye-doped LCP sheets, in which other different latent images had beenpreviously induced on each side. The method allows creating reflectiveand transmissive security features integrated in a single device. Theholographic patterns observed in reflection are completely independentand different from the transmissive images.

Furthermore, the present invention is perfectly suited to the windowsthat are currently being used in certain documents such as national IDcards or banknotes. This invention constitutes an advanced and effectivesecurity feature which, unlike the complex structures of the solutionsof the state of the art in which several retarder layers plus anadditional polarizer layer are required, comprises a single-layer film,in which the liquid crystal adopts variable angles in a circular twistretarder configuration, whose purpose is to align the dichroic dye suchthat it can selectively absorb the incident polarized light according toits orientation.

DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and for the purpose ofaiding to better understand the features of the invention, a set ofdrawings is attached as an integral part of said description in whichthe following has been depicted in an illustrative and non-limitingmanner:

FIG. 1 shows a central vertical cross-section of a device according toone of the embodiments of the invention.

FIG. 2 shows the operating principle of the invention according toseveral steps corresponding to one of the possible embodiments.

FIG. 3 shows an embodiment similar to the preceding ones, but with theparticularity that only one of the holographic elements has beenmetallized.

FIG. 4 shows an embodiment of the invention, where the holographicelements have been selectively metallized.

FIG. 5 shows an embodiment identical to that shown in FIG. 4, except inthis case only one of the holographic elements has been metallized.

FIG. 6 shows an embodiment of the invention, where the liquid crystalsheet has been removed from the confinement substrates.

FIG. 7 shows an embodiment of the invention produced between twosubstrates, from which it is subsequently removed, where the holographicelements have been selectively metallized.

FIG. 8 shows an embodiment identical to that shown in FIG. 7, except forthe particularity that only one surface has been metallized.

FIG. 9 shows an embodiment of the invention in which the reflectiveimages of the sheets adhered to the LCP layer are different from theimages generated in transmission.

FIG. 10 shows an embodiment similar to that shown in FIG. 9, except inthis case a single holographic element has been adhered to one of thesurfaces.

FIG. 11 shows an embodiment of the invention where the LCP sheet is notremoved from the confinement substrates but are left there, forming partof the final structure of the device.

FIG. 12 shows an embodiment similar to that shown in FIG. 11, except inthis case a single holographic element has been adhered to one of thesurfaces.

FIG. 13 shows an embodiment of the invention where the reflective imagesobtained in the sheets adhered to the LCP sheet are different from theimages generated in transmission on the LCP sheet itself.

DETAILED DESCRIPTION OF THE INVENTION

According to one of the embodiments, the device of the present inventionis based on one transparent and colored thin sheet of liquid crystalpolymer (LCP) or reactive mesogen (RM) doped with at least one dichroicdye, though it could be doped with more dichroic dyes. On the sides ofthe sheet there are holographic elements that are partially metallizedor metallized in selected areas. Alternatively, these holographicelements can be completely metallized in part of the device, so part ofthe device is opaque (reflective) while another part is transparent orpartially transparent.

The transparent sheet does not exhibit any image in transmission when itis illuminated with unpolarized light, but when the device isilluminated with polarized light (such as, for example, the outcominglight from a mobile phone LCD or OLED, computer or television display),and observed in transmission, it shows at least one image on each side(depending on the incidence angle of the light in the entrance surfaceand the position of the dye molecules with respect to the impingingpolarized light), the images that are visible when illuminating one sideor the other are completely different. The images provided can be B/W orgrayscale, monochrome or full-color images. To visualize latent imagesin transmission in the transparent device, instead of a polarized lightsource, a linear polarizer can be placed in front of or behind thesheet.

The images that are visible in transmission can be completelyindependent of those that are visible in reflection, or in the simplestembodiment, each image that is seen in transmission corresponds with theimage that is observed in reflection on the opposite side of the film.

This security device therefore comprises, in a single element, level 1features (holographic security features) and level 1.5 features(transmissive features which require an additional element for itsverification, but which is of common use). These transmissive featurescan be considered as level 1 when observed with partially polarizedlight coming from a reflection on any dielectric surface.

The devices according to one of the embodiments of the invention arecreated using two flexible substrates provided with a partially orselectively metallized holographic pattern. These substrates act asconfinement plates and alignment surfaces. The dichroic dye-doped liquidcrystal layer is therefore located between the substrates, althoughalternatively, a dichroic dye-doped LCP layer can be deposited on eachsubstrate, attaching them together face to face at a later stage. Theholographic pattern induces different alignments in selective areas,generating the desired motifs on both sides of the liquid crystal layer.These motifs become visible in transmission by illuminating them withpolarized light. The motifs generated by the holograms themselves arevisible in reflection by illuminating them with natural light.

FIGS. 1 to 13 show a variety of possible embodiments according to thepresent invention.

FIGS. 1 to 5 show examples in which the polymerizable dye-doped liquidcrystal layer is confined between holographic surfaces forming part ofthe final device.

FIGS. 6 to 8 show examples in which the dye-doped LCP sheets areconfined between holographic surfaces that are eliminated at a laterstage, where the LCP sheet is removed and eventually metallized toconfigure the final device.

FIGS. 9 to 13 show examples in which the dye-doped LCP sheets areproduced between two substrates. The LCP sheet is then removed andcompletely or partially metallized substrates with holographic surfacesare adhered thereto. The main difference in the operation of thesedevices and those described in FIGS. 1 to 8 is that, in the new series,the holographic reflective images and the latent transmissive images ofthe LCP sheet are independent of one another, where they can thereforebe completely different.

FIG. 1 shows the central vertical cross-section of a device formed by adichroic dye-doped LCP sheet located between two partially metallizedholographic layers. The confinement substrates (1 and 2) may be rigid orflexible, made up of one or more layers of different materials, and havea holographic pattern on each inner surface. Partially metallized layers(3 and 4) are deposited on the inner surfaces of the substrates. Theselayers can have the same or a different color, reflectivity and surfacecoating. The dichroic dye-doped LCP material is placed between themetallized holographic substrates (5); the material will have twistconfigurations varying between −90° and 90°, depending on theholographic pattern existing on each surface. For the sake ofsimplicity, all the drawings that have been included show patterns withtwo unique perpendicular orientations on each surface; said patternswould only generate black and white images, without grayscale. If allthe possible combinations of grayscales or multiple images are to begenerated on each surface, all the possible orientations of thealignments (directions of the holographic patterns) on each surface mustbe considered.

The device production process starts with the deposition of the dichroicdye-doped LCP mixture on one of the substrates. The sandwich is producedusing the other substrate and cured with UV light. The curing processcauses polymerization of the liquid crystal, as well as polymerizationof the dye, when applicable. Another alternative process for producingthe same device would be the deposition of the LCP layer on each of thesubstrates, subsequently attaching them to one another. In any case,before the polymerization process, the material must reach the liquidcrystal phase for it to adopt the desired alignment.

FIG. 2 shows the operating principle of the device. FIGS. 2b, 2d, 2e and2g correspond to the same central vertical cross-section according tothe structure of FIG. 1. FIG. 2a shows the holographic image that wouldbe seen in reflection if the device is observed from the left side. FIG.2c shows the image that would be seen in transmission if it is observedfrom the left side, illuminating with polarized light from the rightside (with a specific polarization angle). If the incident polarizationangle (or the device) is rotated 90°, the image shown will be a negativeof the preceding one. FIG. 2h shows the holographic image that would beseen in reflection if it is observed from the right side. FIG. 2f showsthe image that would be seen in transmission if it is observed from theright side, illuminating with polarized light from the left side.

FIG. 3 shows the operating principle of a device identical to those ofFIGS. 1 and 2, except in this case only one of the holographic elementshas been metallized. The other element is used exclusively for aligningthe dichroic dye-doped LCP mixture.

FIG. 4 shows the operating principle of a device whose holographicelements have been selectively metallized (6 and 7); in this example,the selected regions of the device present complete metallization. FIGS.4b, 4d, 4e and 4g correspond to the same central vertical cross-section.FIGS. 4a and 4h show the image that would be seen in reflection if it isobserved from the left and right side respectively. FIGS. 4c and 4f showthe image that would be seen in transmission if it is observed from theleft and right side respectively, illuminating with polarized light fromthe opposite side in each case.

FIG. 5 shows the operating principle of a device identical to that ofFIG. 4, except in this case only one of the holographic elements hasbeen metallized. The other element is used exclusively for aligning thedichroic dye-doped LCP mixture.

FIG. 6 shows the operating principle of a device identical to those ofFIGS. 1 and 2, except for the production process. These devices have acentral structure (5) (LCP and dichroic dye) produced between twosubstrates, from which it is subsequently removed. The substrates can berigid or flexible, for one or more uses. Partial metallization of theLCP structure can be performed before or after removing it from thesubstrates.

FIG. 7 shows the operating principle of a device identical to that ofFIG. 4, except for the production process. These devices have a centralstructure (5) (LCP and dichroic dye) produced between two substrates,from which it is subsequently removed. The substrates can be rigid orflexible, for one or more uses. Partial metallization of the LCPstructure can be performed before or after removing it from thesubstrates.

FIG. 8 shows the operating principle of a device similar to that of FIG.7, except in this case only one surface has been metallized. It is alsopossible to create a structure such as the one shown in FIG. 6 with asingle metallized surface.

FIGS. 9 to 13 show examples of devices whose production process isdifferent. First, a polymerizable liquid crystal layer with variouslatent images on each face is produced. Confinement plates inducing analignment pattern on the dichroic dye-doped polymerizable liquid crystalare used in this process. The resulting sheet shows two or more latentimages when it is illuminated with polarized light on any of its sides,or when it is observed through a polarizer. The use of the polarizer isnot actually required: the device works with partially polarized light,such as light originated from a grazing reflection of a dielectricsurface (a shiny floor, table). Polarization component decompensationderived from the proximity of the Brewster angle is enough to show theeffect. It can also be observed by placing the sheet in front of aliquid crystal display, for example, a computer display. The result is athin and flexible sheet containing a set of images. The sheet appearscolored, transparent and uniform when it is illuminated with naturallight. However, when it is illuminated with completely or partiallypolarized light, a series of images emerges. The series is determined bythe face of the device where the light impinges. If the sheet isslightly rotated, a second set of images appears. Different images alsoappear when the sheet is illuminated on the opposite side. The startingmaterial is a mixture of polymerizable liquid crystal doped with atleast one dichroic dye. Confinement plates are used. An alignmentpattern with various orientations is induced on the inner face of eachplate. The alignment directions are parallel to the plane of theconfinement plates. The sheet containing the latent images is obtainedin several steps that are summarized below: first, the dye-doped liquidcrystal is introduced between the confinement plates. The plates orientthe liquid crystal (and therefore the dye) according to the chosenpattern. Second, the liquid crystal is polymerized to permanently fixthe orientation pattern. The polymerized liquid crystal sheet is removedfrom the confinement sandwich. The end result is a polymerized liquidcrystal sheet containing alignment information, where the finaltransparent sheet will show one or more images when a polarized lightsource, a partially polarized light source, or a polarizer is used. Ifthe alignment pattern of each confinement plate is different, adifferent set of images will appear depending on the face that isoriented towards the polarized light source or the polarizer.

The liquid crystal can be doped with one or more dyes. The orientationof the liquid crystal, and accordingly the dye, is determined byconditioning the inner surfaces of the confinement plates whileproducing the sheet.

Glass plates are normally used as substrates in the production ofconventional liquid crystal displays; in this invention, those platesare replaced with confinement plates. The confinement plates can beproduced in any opaque or transparent material, since they are used onlyduring the production process and are subsequently removed.

Another advantageous characteristic of the present invention relates tovariations in the liquid crystal orientation within the plane of theconfinement plates. According to different embodiments of the invention,the orientations are achieved using several methods:

a) Using standard alignment techniques, such as those used in theproduction of liquid crystal displays, but restricting each orientationto specific areas of the plate, forming a pattern. A liquid crystaldisplay usually seeks a uniform orientation over the entire surface. Inthese devices, however, different orientations are generated on the samesurface. Once the liquid crystal has been polymerized, the outerconfinement layers are then eliminated, obtaining a thin flexible sheet.

b) Using interdigitated electrodes oriented in different directions onthe plane of the confinement plates. In this case, electric voltages areapplied during the production process (although they are not requiredduring ordinary use of the device). The electrodes are produced byphotolithographic or micromechanical means, defining the required motif.A liquid crystal layer is subsequently deposited and voltage signals areapplied to the electrodes to condition the orientation thereof.Therefore, an in-plane switching (IPS) effect is generated, forcing theliquid crystal to orient itself according to the predetermined pattern.A multiple alignment capable of reproducing the desired latent images isthereby obtained. Once it is aligned, the liquid crystal is polymerizedin situ to create a permanent pattern oriented within the flexiblesheet. Once it has been polymerized, the electric voltage becomesunnecessary, since the material maintains the orientation induced by thevoltage distribution caused by the electrodes.

c) Using micrometric or submicrometric patterns as described above.Multiple images are obtained by applying several treatments to theconfinement plates. The treatments are applied to different regions ofeach surface. The regions are isolated from one another using differenttechniques: masks, photolithography, isolation barriers, selectivedeposition, thermal evaporation, ink-jet, nano-patterning or any otherstandard microelectronic method.

Any of the described methods produces homogenous liquid crystalconfigurations. The liquid crystal molecules are always orientedparallel to the plane of the confinement plates, although their specificorientation within the plane varies in the different regions of thesurface, such that some areas may become lighter or darker when they areilluminated with polarized light. Several independent images can beobtained on each side of the doped liquid crystal sheet. The variationof orientation in the alignment direction allows to define a grayscaleor even color images. Liquid crystal polymerization allows the sheet,after being removed from the confinement plates, to be usedindependently in many applications: the latent images are alreadydefined.

FIG. 9 shows a device whose operating principle is identical to that ofthe devices of FIGS. 1 and 2. In this case, the reflective images thatare obtained on the sheets adhered to the dichroic dye-doped LCP layerare different from the images generated by transmission on the LCP sheetitself. This does not depend on the method used for creating theflexible sheet.

FIG. 10 shows the operating principle of a device similar to that ofFIG. 9, except in this case a single holographic element has beenadhered to one of the surfaces.

FIG. 11 shows a device whose operating principle is identical to that ofFIG. 9, in which the LCP is confined between two substrates (7 and 8)covered with alignment layers (9 and 10). In this case, the LCP is notremoved from between the confinement substrates but are left there,forming part of the final structure of the device.

FIG. 12 shows the operating principle of a device similar to that ofFIG. 11, except in this case a single holographic element has beenadhered to one of the surfaces.

FIG. 13 shows a device whose operating principle is identical to that ofFIG. 4. In this case, the reflective images that are obtained on thesheets adhered to the dichroic dye-doped LCP layer are different fromthe images generated by transmission on the LCP sheet itself. This doesnot depend on the method used for creating the flexible sheet.

The present invention is directly applicable as a feature for documentsecurity against banknote counterfeiting, or in the authentication ofdocuments, credit cards, checks, packages or any element whose intrinsicvalue makes its verification thereof advisable. The verification isperformed in transmissive mode, observing with polarized light thepattern of dark and light regions that is formed, which dependsexclusively on the orientation of the liquid crystal and the dichroicdye on the entrance surface. If the dichroic dye is aligned parallel tothe polarization, light will be absorbed, obtaining a dark state. If thedye is located perpendicular to the polarization, light will not beabsorbed, obtaining a clear state. If the faces of entry and exit areswitched, the dark and light regions produced will depend on theorientation of the dye on the other side. Therefore, any image can beinduced by forcing the alignment of the corresponding regions on one ofthe sides. The other side may contain a different image, independent ofthe preceding one. The effect is observed by keeping the polarizationfixed and flipping over the device, placing the opposite side of thefilm in front of the polarized light. Alternatively, the effect can beobserved by keeping the cell fixed and placing a polarizer in front ofor behind the sheet.

The use of a polarized light source or a polarizer is not strictlynecessary in order to see the effect. The effect is also seen when thesample is illuminated with partially polarized light, for example, thegrazing reflection from a dielectric surface such as a polished floor ora table. This favors the massive implementation of the invention as asecurity element in labels or banknotes, for example.

The images are observed in reflection with natural light and withoutrequiring any additional tool.

1. A manufacturing method for producing devices for document security including various latent images, characterized in that it comprises the following steps: a) depositing, according to at least one established pattern, at least one layer of at least partially metallized material, forming a holographic element on at least one part of one of the sides of at least one confinement substrate; b) defining different regions on the surface of the at least one substrate; c) inducing different alignment directions for orienting a liquid crystal according to the previously defined regions; d) doping the liquid crystal with at least one dichroic dye; e) placing the doped liquid crystal on at least one confinement substrate, covering the holographic element; f) adding a second confinement substrate, forming a sandwich-type structure; g) polymerizing the liquid crystal, forming a sheet.
 2. The method according to claim 1, wherein step b) further comprises defining the different regions according to the holographic element deposited on the at least one part of the surface of the substrate.
 3. The method according to claim 2, wherein defining the different regions according to the holographic element comprises using micrometric or submicrometric grooves, resulting from the material deposition of step a), for aligning the liquid crystal.
 4. The method according to claim 1, wherein it further comprises removing at least one of the confinement substrates once the liquid crystal has been polymerized.
 5. The method according to claim 1, wherein defining different regions on the inner face of the confinement substrate comprises at least one of the following techniques: photolithography; masks; physical barriers; selective deposition; thermal evaporation; inkjet; micrometric or submicrometric pattern generation; partial (semireflective) metallization which reflects part of the incident light and transmits the rest; selective reflective or semireflective metallization which covers a specific area of the device; or a combination thereof.
 6. The method according to claim 1, wherein generating different alignment directions is carried out in any direction parallel to the plane of the confinement substrates, wherein the alignment directions induced on one of the sides of a substrate are independent of those induced in a second substrate and comprise the use of at least one of the following techniques: mechanical rubbing of the alignment layer; photoalignment of a photosensitive material; oblique deposition of an aligner material; alignment by means of a micrometric or submicrometric pattern; generation of a pattern of interdigitated electrodes oriented in different directions on the surface of the confinement plates; or a combination thereof.
 7. The method according to claim 1, wherein the alignments induced in at least two regions are not linear with respect to one another.
 8. The method according to claim 7, wherein at least two of the induced alignments are orthogonal with respect to one another.
 9. The method according to claim 7, which further comprises determining the relative angle existing between the induced alignment directions to generate different gray levels in the latent image.
 10. The method according to claim 1, which further comprises adding an RGB color matrix to generate the latent images, wherein the color matrix is arranged such that the regions defined in the sheet are made to coincide with the pixelation of the matrix on either the outer face of the already polymerized liquid crystal sheet or on the inner face of a protective polymer layer located on the sheet. 